Polyisocyanates are the important component of the two-part polyurethane product. The different chemical structures of polyisocyanates have a great impact on the properties of the polyurethane, and the polyisocyanates with isocyanurate heterocyclic structure have the advantages such as low volatility, low toxicity and high functionality. The polyurethanes prepared from isocyanurates have the features of high hardness, resistance to high temperature and the like, and by utilizing isocyanurates in polyurethane coating, the hydrolysis resistance, corrosion resistance, thermal stability and dimensional stability of the resulting polymer can be improved. As a very useful curing agent, isocyanurates can be applied not only to polyurethane systems, but also to the systems containing functional groups such as epoxy resin, poly(meth)acrylate, polyacrylamide, phenol resin, etc. In recent years, the application fields of isocyanurates are widening increasingly at home and abroad, and they have been widely applied in the fields of coating, adhesive, sealant, elastomer, foam plastics and the like.
TDI is the main material for synthesizing isocyanurate curing agent in the polyurethane industry. TDI has relatively high saturated vapor pressure, volatility and toxicity, and harmful to the environment and human health. Since TDI as the raw material has not been reacted completely under given conditions, the resulting TDI isocyanurate still contains a certain amount of free TDI monomers. These free TDI monomers not only lead to waste of resources, but also have a lot of adverse effects on the properties of the paint film such as softening of the paint film, and further are very harmful to the health of the workers and the surrounding environment. Thus, these free TDI monomers must be reduced or recycled. In 2010, China has implemented GB/T18581-2009 “Indoor decorating and refurbishing materials—Limit of harmful substances of solvent based coatings for woodenware”, which limits the content of the free TDI monomers in polyurethane coating used for woodenware, wherein the free monomers of the two-part coating after mixing the components at a given ratio are limited to <0.4%, and usually, the main agent:the curing agent:the diluent=2:1:1, thus the standard can be satisfied only when the free TDI % in the curing agent is <1.6%. Therefore, it is important to reduce the free TDI content in the curing agent. Other indexes for evaluating the properties of TDI isocyanurate include color number, xylene tolerance and the like. The smaller the color number of TDI isocyanurate is, the more shallow and high quality of color the paint film can have. The xylene tolerance is defined as the limit for the amount of xylene solvent which can be used to dilute the product without precipitation, which reflects the polarity of the product, i.e., the solubility of the product in the solvent. The higher the xylene tolerance is, the better the compatibility with other components is, and the better the transparency of the resulting paint film is. At present, the domestic TDI isocyanurate products have the platinum-cobalt color number of usually about 30, the xylene tolerance of usually about 1.0, the mass fraction of the free TDI of usually above 1% (based on the solid mass fraction of 50%), whereas the foreign products usually have the mass fraction of the free TDI of usually less than 0.5%. Chinese Patent CN1939949A (“Production of low-free TDI polyurethane curing agent, curing agent therefrom and its products”) disclosed the extraction of the remaining free monomer using a phase-transfer solvent, but the reduced pressure distillation requires a lot of energy, which increases the production cost. Chinese Patent CN1699353A (“low-free toluene diisocyanate trimer with high compatibility and preparation process and use thereof”) disclosed one-step addition of a solvent and a catalyst. The reaction rate is slow due to the low concentration at the start of the reaction, and thus it is needed to increase the amount of the catalyst, whereas the catalyst may be reacted with oxygen, which leads to the deepening of the color number of the product. Furthermore, the one-step reaction might cause a large difference of the properties (for example the physical properties) such as NCO %, viscosity, tolerance, color and the like between the different batches of the products, and cause a large difference of the application properties such as the drying, polishing and extinction properties of paint film, and the compatibility of such TDI isocyanurate with other components between the different batches of the products, and thus the stability of the different batches of the products is poor.
It is needed to develop a stable and reliable process for preparing TDI isocyanurate having low free TDI content and excellent properties of the final product in the art.